Conventionally, a number of vehicle mounted electrical apparatuses operating upon receipt of electric power are connected to lines connected to automobile batteries. In general, automobile wiring is such that a ground-side terminal is connected to a vehicle body frame or the like, that is, the terminal is connected to a so-called body earth.
On the other hand, in recent years, vehicle mounted electronic apparatuses have increased, and lines for communication between a control unit for controlling various apparatuses and an electronic apparatus have increased. Accordingly, power lines and communication lines become necessary for vehicle mounted electric apparatuses, wire harnesses which are wiring lines increase, thus hindering automobile weight reduction. Therefore, as disclosed in Patent Document 1, a power line communication system is proposed as a technique which is capable of reducing the number of wire harnesses by utilizing power lines laid out in a vehicle, as a transmission medium. This Power Line Communication (PLC) uses already-existing power lines, thus eliminating a need to provide a leased line for communication. Hence, the number of wire harnesses in a vehicle is reduced, contributing to automobile weight reduction or parts reduction.
In this vehicle mounted power line communication system, as shown in FIG. 7, a line 12 for power feeding is connected to a positive pole side of a battery 10, and a ground terminal which is a negative pole side of the battery 10 is connected to a body earth 14 such as a vehicle chassis. Λ number of vehicle mounted electric apparatuses 16 are connected to the line 12 at the positive pole side, each of which has its own function. In addition, a plurality of communication apparatuses 22 are connected to a communication line 18 branching from the line 12, via a predetermined frequency filter 21. The filter 21 increases the impedance of communication lines in a power line communication block 20 connected to the communication line 18, preventing a high frequency signal employed in power line communication from propagating to a non-power line communication block 24 of an electronic, apparatus which does not perform power line communication.
In addition, as shown in FIG. 8, with a negative-pole power line being connected from a body earth, the communication line 18 is laid out in a pair of positive and negative lines.    Patent Document 1: Japanese Laid-open Patent Application No. 2004-56197